Double seams have been in use by can fillers and can makers in order to ensure a high quality seal inside a metal container, which helps provide longer durability of their contents and a higher degree of separation from environmental hazards and contamination.
The double seam is typically performed inside a seamer, typically by having a cover locked into a chuck, and then the cover and a can flange are folded twice into a double seam.
In order for the seal to remain secure, the double seam closure must be maintained all around the can. This is of critical importance, so anyone closing seams (can fillers as well as can makers) and some recipients of enclosures, which contain closed seams must make routine checks to ensure these seams are sealed properly.
The FDA presently requires at least one test set to be performed at intervals of once every 4 hours. These tests require taking a complete set of cans from a seaming line—at least one sample per seamer head; Seamers can contain between 1 and 24 individual heads) from a production line, at different stages of the double seam production.
The tests include either or both non-destructive tests (NDT's) and destructive (teardown or sectioning) tests.
Destructive tests typically involve a teardown procedure, which uses a can stripper to remove the cover-hook from the can and allows for visual inspection. Additional destructive tests include cutting through the seam at 1 to 4 sections at regular intervals around the can, and inspecting the image of the cut cross section of the seam. These types of destructive tests provide valuable information regarding that particular measurement section, such as overlap, body and cover-hooks, body and cover hook butting, free space as well as seam length and thickness.
Additional issues that can occur are various issues like false seam (where the closure process misses or slips and the seam is actually open in some areas), vee, droops, asymmetrical closure (the overlap hangs to one side), small overlap, and large free-space (the seam is too “open”). Other cosmetic issues, such as sharp seams can be cosmetic but do not interfere with the closure's seal.
However, the cutting itself can warp or extend the cut material such that the measurements contain artifacts such as burrs. Moreover, only a few cuts may be made per can without losing can integrity.
Non-destructive tests may include pressure testing (typical strain gauges as part of the seaming process), X-ray measurements, and external measurements, such as using a caliper, or specially designed gauges—the latter typically serving for measurement of countersink, seam thickness (also known as seam width) and seam length (also known as seam height).
One double seam measurement system is described in U.S. Pat. No. 4,600,347 to Continental Can Company, Inc. (US), in which part of a cam of a double seaming apparatus is deformable. Strain gauges monitor the deformation of this part of the cam track and the signal from the gauges is processed to identify abnormal conditions together with details of the specific force, relevant machine and date/time of each abnormal condition.
U.S. Pat. No. 7,736,112 to Crown Packaging Technology, Inc. (US) describes another double seam monitor device for measuring the strain/force applied by a lifter cam so as to provide information interpreted to indicate seam quality, gross seam defects and seaming machine condition. The apparatus includes: a lifter mechanism for lifting the can body; seaming tooling, and a device for measuring the strain of and/or force applied to the lifter mechanism by the lifter cam.
The CSG-2073 gauge by CMC to CMC Kuhnke GMBH (DE) is described by the manufacturer as a semi-automatic gauge equipped with double seam thickness and optionally double seam height measurement systems, in which after a can has been placed into position, an LVDT (Linear Variable Differential Transformer) gauge measures at up to 50 points (around the diameter of the can).
WO2010048914 to CMC Kuhnke GMBH (DE) describes a positioning device for analyzing a double seam cross-section using X-rays, including an X-ray source, having a first stop associated therewith for a can to be analyzed in the region of the outlet opening for the X-rays, and detectors for receiving the X-rays. The can to be analyzed is clamped relative to the X-ray source by way of at least three stops disposed offset from each other, wherein the positions of two of said stops can be actively varied in order to perform an adjustment.
However, X-ray systems measure at only a few points around the can, and pressure systems on the seamer can miss even obvious issues because of their technology (e.g., false seams, for example when the body and cover edges are not folded into each other).
A very serious disadvantage of many double seam tests is that a very small section of the double seam is sampled around the can. This works as long as the seam is consistent around the circumference of the can. However, when there are local defects in particular areas, this method will most likely miss them, which means that the tests do not ensure that the can being tested is, in fact, sealed—many tests would have to be conducted until the problem is identified as consistent. Another alternative is to increase the testing rate (from the mandated once per 4 hours to round the clock testing, or even real time testing), but such methods will still test one or more sections of the can and not the entire circumference of the can itself. Local defects include for example composition buildup, seam skids, bumps, dimples, wrinkles Bad seamer bearings, cracked tooling and seamer setup issues (such as loose setup, loose shafts or shanks) can cause issues that may only be apparent by inspecting the seam all around the can. Not just peak points outside the specifications are of issue, but also range issues that can identify that the seaming process is oscillating.
Considering that beverage cans can reach as much as 4000 cpm in production environments, as well as the trend to reduce can end diameters and material thicknesses, the seaming process is now far more prone to manufacture errors that can be missed. Moreover, control limits have been reduced accordingly, as part of the initiative to reduce wall thicknesses and narrowing of can necks.
Even with increased test rates, a problem can nevertheless be missed due to small localized flaws that would result in many cans during production simply being bad. With Just in Time production, the distance from manufacturing to distribution is shortened. If a local flaw results in cans going out of the control limits even at a small area around the seam, this issue can be discovered many hours after cans were being produced; Since cans are shipped out almost immediately after being production and testing, this could result in massive recalls.
It should be noted that cans with micro-leaks or that do not have a tight enough seal can expose the consumer not just to eating spoilt or degraded food, but can also expose the contents of the can to harmful industrial chemicals, such as anti-fungal sprays and fluids, that are used near where cans are stored. Cans that are exposed to such chemicals without being completely sealed could result in food poisoning.
Most if not all commercially available seam measurement systems, such as various calipers applied to the seam walls, e.g. feeler gauges, are not capable of accurately positioning the system according to the chuck wall angle. In some systems a manual and/or other adjustment is made to adjust the system orientation according to the chuck wall angle at each point of measurement or even set to a fixed value. Apparently, the angle does vary somewhat around the seam of a can (particularly if there is a defect in the chuck, in the bearing, bushing or orientation of the can), and from can to can in a batch. It is very difficult and lengthy to make a correct orientation, and incorrect measurements in this respect, are both inaccurate and imprecise. Moreover, the delicate and flexible nature of double seams, in particular in beverage cans, can further result in different operators obtaining completely different results for the same measurement point. Some other systems allow fixating a measured can such that the can is at a presumably constant angle (or modifiable angle, that is changed manually) relative to a reference area on the measuring system. Nevertheless, such systems also are inaccurate and imprecise.
The invention aims to provide improved measurement of double seams. The objective is to reduce the number of missed local problems by fully inspecting each can.